In a process of molding a resin (plastic) product, a glass product, a metal product or the like with a mold tool, the product may be hard to release from the mold tool because of friction between the product and the mold tool. As known, the release resistance relates to the surface roughness of the mold tool. In the following, this will be described, taking resin molding as an example.
FIG. 1 is a graph showing relationships between the surface roughness and the mold tool releasing force (release resistance) shown in Non-Patent Literature 1. As shown, the release resistance decreases with the surface roughness (average roughness) Ra until the surface roughness Ra reaches about 0.2 μm. However, in the range of the surface roughness equal to or lower than about 0.2 μm, the release resistance rapidly increases as the surface roughness decreases.
The reason why the release resistance decreases with the surface roughness in the range of the surface roughness Ra higher than about 0.2 μm is that the frictional resistance caused by the anchoring effect of the resin penetrating into the irregularities on the mold tool surface or digging of the resin by sharp projections on the mold tool surface decreases. However, in the range of the surface roughness Ra equal to or lower than about 0.2 μm, the adhesive force between the solid surfaces probably increases and becomes predominant in the frictional force to increase the release resistance.
In the range of Ra equal to or lower than 0.2 μm, small gaps having sizes of several hundreds of nm or less are formed between the irregularities of the mold tool surface and the resin surface. If a liquid exists in the small gaps between the solid surfaces, a liquid cross-link referred to as a meniscus occurs due to capillary, and the attractive force (meniscus force) provides a strong adhesive force between the solid surfaces.
With the recent trend toward smaller and more precise electronic or optical components, the demand for the average roughness Ra of the mold tool surface equal to or lower than about 0.2 μm has been increasing. Therefore, the adhesive force or the meniscus force of the thin liquid film formed by aggregation of water or oil in the release agent (if a release agent is used) or the atmosphere, which poses no problem when the mold tool surface roughness is high, has become problematic as a factor of increasing the release resistance.
It is known that the adhesive force due to the meniscus can be significantly reduced by forming fine irregularities on the surface. Patent Literature 1 discloses an art of reducing the release resistance that involves mirror-finishing the mold tool surface that comes into contact with resin until the surface roughness Ra is equal to or lower than 80 nm and then forming a required number of fine recesses or projections on the surface in a required arrangement. A desired releasability is achieved when the fine recesses (projections) having widths ranging from several tens to several hundreds of μm and aspect ratios of 1 or less are formed at intervals ranging from 10 μm to several hundreds of μm.
According to another common method of reducing the release resistance, the adhesive force due to the meniscus is reduced by sparsely making shallow scratches extending in the release direction to form fine recesses after polishing the mold tool surface that comes into contact with resin until the surface roughness Ra is equal to or lower than about 0.2 μm.    Patent literature 1: Japanese Patent Application Laid-Open No. 2008-307735Non-patent literature 1: Yoshikazu Kobayashi, Kenji Shirai, Tetsuo Sasaki, “Relationship between Core Surface Roughness and Ejection Force for Injection Molding”, Journal of Japanese Society of Precision Engineering Vol. 67 (2001), p. 510